Over the course of history humanity rode several kinds of animals to war: horses, camels and elephants being the most common. However, bears are not amongst them.

So I was wondering, if they could be properly domesticated and/or trained, would bears be feasible as ridable animals in wartime situations?

A bear is a lot more fierce than a horse or dangerous than a camel, but less big of a target than an elephant. This of course also has to take things into consideration like being able to charge, marching speed, their diet, endurance and other such things.

$\begingroup$@Ewan Dogs are not bears, so that's irrelevant to the question at hand.$\endgroup$
– Thomas JacobsMar 27 '16 at 11:18

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$\begingroup$I would like to mention that using elephants as cavalry was anything but common. Only really sophisticated and complex societies were able to sustain War Elephants.$\endgroup$
– mathgeniusMar 28 '16 at 16:18

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$\begingroup$@Ronk They aren't, actually; they're in the Ursidae family. They do share the Caniformia suborder with the Canidae family, which is perhaps what you were thinking of, but so does a walrus, so calling them "canine" based on that is a bit of a stretch.$\endgroup$
– azurefrogMar 29 '16 at 3:38

Profile. The war bears would have a better rider-supporting profile. Horses and camels allow the rider's legs to stay relatively stable, and an elephant is rigid enough to allow a howdaw. The bears need to bear a saddle of sorts. Give some thought to the design of this saddle. After all, if it is a bear cavalry, it is assumed the rider fights too, and not just steers the bear.

Diet. fgysin suggests eating the fallen enemies. I think that if this is done, it should be ingrained into the culture, into a sort of ritualistic proceeding of cleaning up the battlefield. Just allowing the bears to ravage the fallen is bad, since making the bears eat people can teach them how delicious they are. see #1.

Diet yet again. Maybe the bear cavalry works in your world because of some rare protein-rich plant that can be grown to feed the bears. This plant might be mild poisonous to humans, but bears can eat them without any issues.

$\begingroup$@mathgenius yeah. addressed in bullet #4. Ritually eating the fallen might be a tribute to the bear god and the eaten will be rebirth as bear-demigods-enheriar-mecha to fight in the final battle against the ranks of the damned alongside mighty wargod Kratos and that is a honor because after ragnarok they will be awarded... yadda yadda yadda. by any civlised (sic) species seems a bit too earth-like when you are designing a fantasy world.$\endgroup$
– MindwinMar 28 '16 at 17:10

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$\begingroup$@Mindwin, I did see #4, but I felt the need to note the cultural impact such an action will have, apart from the dietary effects. You have a point about anything in a fantasy world being too earth-like, HOWEVER, please note the reverse occurs too often (for my taste), which is saying that "because it is a fantasy world" everything is allowed and you can do as you please. The truth, imho, lies in the golden middle - while a fantasy world is just that - a fantasy, we draw parallels from the real world. The primary untainted parallel we always make is behaviour of sentient creatures.$\endgroup$
– mathgeniusMar 28 '16 at 17:32

Traditionally when an army ran low on supplies, the men would start eating the mounts. When an army with bears starts to run low on supplies, the mounts would start eating the men. This could be really bad for morale.

$\begingroup$@spark Bears will even eat each other when they're hungry, I doubt domestication would stop them eating us. Ultimately they'll remain bears.$\endgroup$
– SeparatrixMar 21 '16 at 12:10

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$\begingroup$@spark The argument here is that the exact trait that makes them desirable mounts (their ferocity in battle) makes them undesirable mounts (unless you breed that ferocity out, they'll be a liability). I would agree with that argument.$\endgroup$
– CronaxMar 21 '16 at 13:16

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$\begingroup$@fgysin dogs are pack animals, bears are not. It's a very important distinction that I explicitly made.$\endgroup$
– SeparatrixMar 21 '16 at 13:55

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$\begingroup$and the reason that is important is pack animals view pack members as family and (usually) won't turn on each other even when starving. Bears are loners and have no such inhibitions.$\endgroup$
– Jim2BMar 21 '16 at 15:45

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$\begingroup$The morale of the mounts, though, would be higher...$\endgroup$
– OldcatMar 21 '16 at 17:48

The key reason why this is not feasible, is down to endurance - Horses, Elephants and Camelids are native to areas with large plains, and migrations - they are effectively built to last for travel between points - which makes them ideal as beasts of burden.
Bears, on the other hand, are built to conserve energy - they are (mostly) native to areas where there is more cover, and even if they are not a hibenatory species, they will go into a torpor to ensure less energy is used when little is available.

If you want an animal which would have the intimidation factor of a bear, and the endurance of a horse - I'd suggest an Auroch...

$\begingroup$I also see endurance as the primary inhibitor to bear cavalry. The hallmark of cavalry is the ability to cover long distances quickly, Bears are fast in sprint (terrifyingly fast) but they can only sprint.$\endgroup$
– GreenMar 21 '16 at 13:02

$\begingroup$But bears would be better for short-distance cavalry combat , seeing as they already have claws and teeth and are adapted to use them , whereas horses , camels do not . Of course , elephants are out of the question$\endgroup$
– sparkMar 21 '16 at 13:16

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$\begingroup$@Green: In most cases your cavalry will not be riding alone, but will be operating with infantry support and a large supply train behind. While bears would not make good scouts/skirmish cavalry, they could certainly hold their own as heavy cavalry in your army.$\endgroup$
– fgysinMar 21 '16 at 13:42

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$\begingroup$@fgysin, that is an excellent point. I had forgotten about heavy cavalry.$\endgroup$
– GreenMar 21 '16 at 13:43

As the OP talked about domesticated animals I think bear cavalry is feasible overall.

I think domesticating bears should work out quite fine: bears belong to the same suborder as wolves/dogs, which were domesticated very successfully (and were also used in war, to a certain extent!). Also bears have been famously displayed in various circus/movies/shows over the ages, showing that even the wild bears out there can be tamed up to a certain degree.

Some additional points:

Endurance

As mentioned endurance could be an issue. However, cavalry normally doesn't have an exceptionally high marching speed when compared to infantry, so while your bear cavalry will be slower than horse cavalry, it should easily keep up with infantry regiments and your supply train (which you would need to bring anyway). In combat cavalry is usually used for relatively short charges, which a bear is very capable of. So bottom line: bear cavalry would form a heavy cavalry unit, but you'd still need horses for scouting/skirmishing/surprise ambushes, ...

Food

Food is an issue. Bears eat a lot, and they mostly eat things which you will not find sitting on the roadside by the wagon load (like, for example, grass for feeding horses). So you'd need a good supply train to feed your bears. The good news is that bears are omnivores, so they'll eat pretty much anything... You could probably come up with some sort of special bear pemmican which is durable, compact and can be manufactured in bulk. Also after winning a battle meat is back on the menu, boys! ;)

Tactics

I think your bear cavalry will need to come up with some new tactics. While armoring bears and using them to charge into enemy formations should be feasible, I don't think it really uses the bears to their fullest potential. Bears in fights often stand up on their hind quarters, and use their front paws for fighting as well. The problem here is that this won't work so well for the rider... Maybe the bear cavalry would charge into the mêlée, where the rider would jump of and fight side-to-side with his mount.

Fighting

This is really where your bear cavalry will shine. Have you ever seen a bear fight an other bear? Or heard the stories of bears which kept attacking their victims even after being shot multiple times? Bears are very strong and very durable. Horses are natural flight animals, and while they can be trained for combat, the just don't have the same potential. I feel that a domesticated bear trained for combat (and maybe armored?) could be a ferociously efficient fighter.

$\begingroup$Yours should be at the top$\endgroup$
– Aarthew IIIMar 21 '16 at 14:53

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$\begingroup$The issue with domestication of bears is that, unlike wolves/dogs or horses, they are not pack/herd animals, so there's no way for the humans to tap in to that existing social organization.$\endgroup$
– jamesqfMar 21 '16 at 16:07

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$\begingroup$@jamesqf: I don't see an necessary connection between herd/pack animals and their 'domesticate-ability'. Do you have a source for this? Chicken/cats were domesticated, although they are not herd animals. Ferrets also were domesticated, and used for hunting. Pigs/cows are herd animals too, but aren't as easily trained as a horse or dog for example. Bears however, have been shown to be trained well on occasion.$\endgroup$
– fgysinMar 21 '16 at 18:51

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$\begingroup$@fgysin: Ever try to train a cat? Or a chicken? I mean reliably, the way a dog or horse is trained, not occasional "do tricks in the circus" by a professional trainer. There's also major differences between the sort of evolved partnership there is between humans and dogs/horses, domestication in the sense of keeping for food, and occasional training of what are essentially wild animals like lions, tigers, & bears.$\endgroup$
– jamesqfMar 21 '16 at 20:48

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$\begingroup$@jamesqf This is a fictional world so the OP could always just decree that the bears in his world are pack/herd animals. If that is the biggest concern then it should be easy to work into a story a sighting of a herd of bears.$\endgroup$
– ErikMar 21 '16 at 22:07

Bear cavalry is a no-go for about the same reason that we do not raise bears for meat. After all, a bear can be fed with just about anything, and grows quickly, and thus should be a good source of food. But it has a big drawback, which is that bears don't tolerate well other bears. You cannot make a herd of bears (however awesome it would be). There is a nice analysis of the conditions of domestication of animals for food production in Jared Diamond's Guns, Germs and Steel, which cites several animals as counter-indicated, for various reasons. Bears are the example of an animal that you cannot put in groups.

Of course there are other reasons why bear cavalry would be challenging, as cited in other answers: management of hibernation, difficulty of fitting a saddle on an animal whose weight can triple over the year (they really thin a lot during hibernation), possible low tolerance of the spine to the pressure from a rider... but all of these could probably be overcome. The impossibility to keep bears together is a much bigger problem, which prevents assembling a significant bear cavalry squadron. At best, you could have a couple of scouting bears.

(Bears are remarkably good at moving across difficult terrain, so they could make good scouts in mountainous areas.)

Edit: I thought I would add a few extra details. We must make a difference between species.

Polar bears (Ursus maritimus) are mostly carnivorous, and very hard to tame; they see humans as a potential food source (they prefer the taste of seals, but they will still happily munch on human meat). There is currently, to my knowledge, a single tame polar bear in the World; she is used occasionally for movie shots, but even though, they keep her out of fight scenes in case the instinct takes over (I read these details from this book).

Brown bears (Ursus arctos), including grizzlies, are your best bet for taming. They are huge but relatively gentle. Their diet is also at least 75% plants, and they are really not picky. They are also very tolerant to long periods without food, because that's what they do when hibernating. A grizzly can behead a human with a single paw blow, but it usually won't bother, and does not crave human meat.

Black bears (Ursus americanus for the American species, and Ursus thibetanus for the Asian one) are smaller than brown bears, and a bit more irascible, so they would be second choice for cavalry. They might be more useful as enlarged versions of attack dogs; they also have a very keen sense of smell so they could possibly be used for detection rather than charging.

Giant pandas (Ailuropoda melanoleuca) need fresh bamboo shoots. They are totally inept at many things, and it is very puzzling that they have not gone extinct already.

The other kinds of bears (spectacled bears, sloth bears, sun bears) are smaller and mostly tree-dwelling; they won't be comfortable on the ground, which makes them unsuitable for military purposes.

A lot of people have mentioned that endurance is a problem in bears, but the truth is that what evidence there is suggests otherwise. I won't be answering the question completely here, but I will discuss the endurance of the bears.

Let's first establish their suitability over short distances. This source says that grizzlies:

have considerable endurance, for covering two miles at from 25 to 28 miles per hour proves a stamina that would certainly try the best of horses.

Similar sources mention observing grizzlies running at that speed for similar distances without obvious signs of exhaustion. Compare to horses, which have a similar maximum speed and will be exhausted after running 2-3 miles at it.

So as far as we know, there isn't much of a reason to assume that bears don't have suitable levels of endurance. Assuming that the bears have been bred and domesticated, it's likely that they'll have good stamina.

The main issue I can see with a domesticated bear cavalry isn't about how suited they are for the role as a creature, but rather the logistics of keeping them fed. Larger bears typically eat a lot more than horses (where horses might eat 20lb of food a day, and smaller black bears up to 18lb, but brown bears and grizzly bears might eat 90lb a day.

However, this might be related to building up reserves for hibernation; if this is controlled or bred out of them, they may eat smaller amounts and not hibernate.

Bears may also need a more varied diet than horses, complicating things further.

$\begingroup$Your profile pic has never been more appropriate.$\endgroup$
– Rand al'ThorMar 21 '16 at 23:09

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$\begingroup$It's not just weight of food, but the nature of the food. Horses are grazers, eating mostly grasses, so horse cavalry can be fed with (relatively) easily stored hay & grain, or can often live off the land. Bears, being omnivores, would likely require a similar diet to humans, so the army would have to store & carry provisions for them, increasing the logistics train.$\endgroup$
– jamesqfMar 22 '16 at 16:29

I'm going to avoid some of the issues which have already been covered by some of the excellent answers upthread, and focus on the key issue: you want bear cavalry. Bears are not well shaped for riding, have a strange "rolling" gate when running (which would make riding a real challenge) and tend to rear up to fight with claws and teeth, which negates the true benefit of shock cavalry, the momentum of the charge. Having your mounts pull up and rear to fight makes most cavalry weapons like lances, spears and sabres far less effective than they are with cavalry mounted on horses.

You would be effectively reduced to the sorts of weapons that ancient cavalry used (javelins or bows). The Ancient Greeks had a form of light infantry trained to grasp the manes of the cavalry horses and run alongside them into battle (you see this trick in the movie "Alexander", when they suddenly emerge from the clouds of dust raised by the Companion Cavalry and attack the Persian Cavalry sent to stop the Companions), so bears might be accompanied by this sort of infantry soldier rather than being ridden like a horse.

So a bear charge will see bears racing into the enemy lines accompanied by light infantry wielding swords. If the emery line has been breached by missile fire (archery etc) or a Cavalry charge, then a well timed rush by bears and their infantry will create havoc and open the breach further for exploitation.

$\begingroup$Some good points about the combat system. Riding will definitely be more of a challenge. For the lancers, you the bears could be trained to stay low into the first charge, after which lances are abandoned. You could also see a lot of riders being archers rather than melee fighters, using the bears initially for positioning to shoot, and having the bears break ahead of the riders to engage incoming opponents.$\endgroup$
– SamthereMar 22 '16 at 14:34

$\begingroup$THANK YOU!! I can't believe I had to get this far down the list of answers to see "bears run weird and it would be nearly impossible to ride". Personally, I find this to be the most relevant factor.$\endgroup$
– Jacob ZimmermanMar 23 '16 at 16:12

The bear would be very fast and much more powerful than the horses or camels (having claws and very sharp teeth) so the bears could be taught to fight along with their rider . Of course , if you ever have to face an elephant in combat , your bear and you are gone ( see this ). But otherwise , the bear would be a feasible choice for combat . To feed them , you would need fish or meat which is costlier than grass/hay which you could use to feed horses . This means the bear would be more expensive to keep. This would mean that bear cavalry would be less preferred than horses / camels . Bears are quite fast , being able to outrun a racehorse for short distances , but cannot endure at such speeds for very long (source http://www.bearsmart.com/about-bears/general-characteristics/ ) .Hope I helped !

EDIT - Another problem is - BEARS HIBERNATE! . So , if war breaks out in winter time , when your bear cavalry is hibernating , better have backup ! This probably means bears can only be troops in summer and definitely not your primary cavalry .

$\begingroup$First off, the 'hibernation' of bears has been in question for a number of years now because they don't behave anything like other hibernating animals. Secondly, hibernation is a survival technique, not a design feature of the animals, though instinct will play a role. Most non-pregnant or non-older bears in captivity rarely hibernate as they have access to regular meals and the like so there is no need for them to conserve. Likely 'domesticated' bears being used as mounts would not feel the need to hibernate so only instinct might drive them towards that, which can be changed over time.$\endgroup$
– Prof. BearMar 21 '16 at 12:54

$\begingroup$@AarthewIII if I recall correctly, bears that have a diet of mostly fruit and berries have a tastier meat than those that are on a mostly meat/fish diet$\endgroup$
– Nikita AkopjansMar 21 '16 at 15:41

$\begingroup$Grizzly and black/brown bears are only opportunistic carnivores. They get most of their calories from roots, bugs, grubs, berries, and some leafy matter. I bet you could come up with a reasonable food for them based on cooked cracked grain, supplemented with a small amount of dried fish. FWIW sled dogs in the klondike were fed 1/2 a dried salmon, or smaller amount of salmon and boiled rice. Rice was easier to carry, but required cooking.$\endgroup$
– Sherwood BotsfordJan 11 '18 at 17:12

Bear Saddles

The other answers here are excellent and cover a lot of the potential pros and cons of bear cavalry, I just want to add a suggestion for a saddle that may help alleviate some of the issues you may encounter.

First, what won't work:

A horse style saddle: A horse saddle is fine while the horse is horizontal, but when a horse rears up on its hind legs, riders tend to fall off. Since one of the best advantage of a bear is that it can rear up and fight with its bite and front paws, you want a saddle that can hold a rider during rearing. Additionally, bears are too wide to have the legs easily splayed to either side of the body the way you can with horses.
An elephant-style saddle: An elephant saddle is great because it is basically a whole platform for a person to sit/stand on, and it very stable. Unfortuantely bears just aren't big enough for that kind of saddle to be practical. Alternatively you can sit on the neck of an elephant, but that puts you in a bad position on a bear (if they could even comfortably hold you).

Here's my suggestion:

A saddle where the rider has his legs to either side with his knees forward and his feet back (the legs would make a "W" shape). His feet would fit into a stirrup that holds it securely both while the bear is horizontal and while it is vertical. This would look like the bottom part of a horse stirrup, but turned 90 degrees and attached securely to the saddle. The knees would have some sort of padded spot to rest on while in the horizontal position to bear a lot of the rider's weight. There would also be a good handle of some sort in front of the rider where a horse saddle's pommel would be, but with a horizontal grip to hold onto. The saddle would wrap around the middle of the bear as well as around the shoulders so that it is secure in both orientations.

Advantages of this design:

When the bear is charging the rider can hold onto the grip securely and stay low on the saddle (the leg position encourages the rider to say low and secure).

When the bear is reared up, the rider can stand up in the saddle as well (now with legs extended and holding onto the handle).

In either position the rider can use a one-handed weapon to fight against the opponent's bears and riders.

Disadvantages of this design:

It would likely be uncomfortable to those not used to it. The leg position is totally doable for flexible people, but would take a lot of time to adjust to if you weren't very flexible. It is possible the discomfort would be too great, it is hard to know without trying it (Note: this is not recommended before you actually tame/domesticate the bears).

Mounting may be difficult, but I suspect that you could add a footstep on the side of the saddle to aid in getting up and down.

If anyone who is more artistically inclined would like to mock up a drawing of this kind of saddle please do and send me a link, I would love to include it

It expands on why horses got domesticated but not... zebras. The TL;DW version is that, well, horses form docile herds with a lead horse, whereas zebras are stubborn and all over the place. Catch the lead horse and the herd is basically yours. Rinse and repeat for cattle, sheep, etc. Hence we domesticated horses but not zebras, cows but now buffalos, etc.

Now try to recollect when you saw a docile herd of bears with a lead bear steering it. :-) There's more to the video but that alone rules bears out.

(And no, I do not think it's realistic. But hey, it's World Building...)

$\begingroup$The zebra thing is wrong! It assumes that we domesticated the horse in one step: wild animal to riding animal. Instead we went wild horse to tame animal for meat (like a beef cow) to draft animal (wagons & chariots) to riding animal. Also plains zebras and mountain zebras have the same social system as Przewalski's horses and domestic horses. Grevy's zebras are the only weird ones. And if we can domesticate something as bad tempered as wild cattle or camels, zebra would be a push over. Watch this BBC News video on psycho Heck cattle: youtube.com/watch?v=RSGiwXl8jzE$\endgroup$
– DrBobJun 29 '16 at 6:34

In terms of feasibility and desirability, it might be more preferable to breed 'up' a species of animals which is already domesticated.

Like dogs.

Mastiffs, wolfhounds, and great danes were all bred for use in battle. While they're not large enough to ride, it's conceivable that over successive generations they could be bred up to be larger still. Consider the Pomeranian, a small dog breed sometimes smaller than the chihuahua. A century or so ago, they were bred down from larger Wolfspitz work dogs. A breed fostered over several centuries for size, and already starting from a larger dog, might very well be able to be large enough to ride.

For fiction, bears would suffice, I suppose. But they do not have a skeleton which is ideal for riding, and their lope is quite uneven. Most bears, moreover, are probably not large enough to ride as mounts. Consider: a typical riding horse weighs about 1200lb, and can only carry about 250lb. The largest of male bears only get up to that weight, whereas they're typically closer to half to a third of that. You're not carrying a 200lb man with 50lb of gear on a 400lb bear.

One thing not addressed by previous answers, but worth considering, is that bears are very intelligent animals. In fact, both horses and bears are very intelligent (possibly as or moreso than dogs): This is what makes them easy to train. But it also begs the question: what incentive does the bear have?

A horse forms a very close bond with its rider, in part because it is a social animal and its social dominance heirarchy enables it to interact with dominant members of the herd (humans, especially its rider; with dogs, this is sometimes seen as a parental relationship, since wolves operate in family pack units and dogs are widely considered to be infantilized wolves). Horses' social dominance heirarchy and pack/herd mentality allows it to respond to threats (charging, rearing up) as if it were challenging a competitor or protecting the herd, differently than it would respond alone in the wild (running away in terror). In short, horses feel protected.

They also become docile when subjected to domination, even mistreatment (sadly). That is why they tolerate being ridden, long term.

Now consider the bear: even in a scouting expedition (for which the bear is ideally suited), what incentive does the bear have to suffer the harsh conditions of war alongside its master? The bear does not need its master for protection such as from carnivores, which is the key social contract between men and horses; or rather (unlike the dog) it is not familiar enough with human technology to understand that being with its master will protect it from the ranged weapons of rival humans; in the absence of ranged weapons, a bear is capable of fighting and scouting on its own: the rider is merely dead weight.

The bear would respect its master but not be led around by him, certainly not into battle. Brown are one of the few species that don't immediately respect men as the apex predator and aren't dependent on them for protection from other predators; when the human thinks "we're in trouble", in the (apocryphal) words of Tonto from The Lone Ranger, "What do you mean 'we', human?"

Domestication is not the same thing as taming. All domesticated species have a social hierarchy, where a human replaces the alpha animal of the herd. Therefore, domestication requires social animals, i.e., wolves, horses, cows, sheep, goats, etc. They all have a degree of hierarchy in their social structures.

Taming is when you reinforce an animal to do something. You can reinforce a cat to litter in a box but you cannot reinforce it to follow you unquestioningly. So, even if you achieve selective breeding to weed out non-conforming genes, there is no initiative for the bear to follow you. Therefore, the best you can get out of a bear is a huge cat.

Can you make a cat attack someone? Sure, I have witnessed cats attacking on command. However, battlefield is much more than attacking on command. Fighting in formation is the most important thing on the battlefield. Well trained horses and camels actually bite and kick enemy soldiers and animals. Well trained elephants swing their tusks to break spear walls. These things are possible only with domestication.

A similar comparison can be made between gladiators and legionaries. Gladiators were trained to fight single combats with certain rules, while legionaries were trained to fight in formation. While most gladiators were good enough to defeat any legionary in single combat, use of gladiators in medieval warfare has failed miserably each time.

If you are looking for legendary animal fighters, orcas might offer better results. Some large dinosaurs are suggested to be hunting in packs, but this doesn't necessarily mean they have tight social hierarchies. Nonetheless, they are more promising than bears.

$\begingroup$(1) What’s a “1v1 battle”? Do you mean “one-on-one” (or “1 vs. 1”)? (2) Am I missing something? Is there a land animal called “orca”? How could whales be used in a cavalry?$\endgroup$
– Peregrine RookAug 25 '18 at 19:33

$\begingroup$@PeregrineRook Orca social dynamics might allow them to be used in battlefield. So, you can theoretically use them to fight in formation in naval warfare. I omitted this because I thought it was obvious. I fixed the 1v1 battle. Thank you for that.$\endgroup$
– C.KocaAug 25 '18 at 19:42

Some points on bears in warfare, as well as an alternative to bears which would likely be more favorable logistics wise. Oh, and apologies for the lengthy answer.

To start with, how well would a bear work out in war?

To start off with, you could armor a bear. A bear that was kept for war would logically be well fed as it grew, and so would be a good size, note, that means it would be about 1.5 metres at the shoulder on all fours, and 3 metres at the shoulder while on its hind legs.

I took a look at a picture of a rearing grizzly, assumed a height of 3 metres at its shoulders, and an average armor thickness of 2.25mm (the thickness of plate armour was 1.5 - 3 mm, thicker in the areas more likely to be hit, thinner in other areas). If you gave a bear of this size armour to cover it's entire torso, the top three quarters of its arms, the top three quarters of its legs, and a kind of collar protruding from the breast plate to protect the throat, you would use just shy of 90kg of steel. A bear of this size can lift up to 500kg, so this is not a problem.

An unarmoured bear is already difficult to injure, and it is important to note that weapons don't cut steel. When facing an opponent armoured in plate, you aim for week points, areas of the armour which need to be flexible, in this case, the armpits, which can be protected with pauldrons or besagews, the elbows, and the backs of the knee's.

This would be difficult to do, your average polearm tips out at a length of 1.8 metres, but only half of that extends out in front of your hands, and a medieval man had an arm of about 75cm. This gives a theoretical range of about 1.65 metres, but you don't use the tip of a weapon, and you don't fight with your arms fully extended. All things considered, the average soldier armed with a polearm, can probably fight at a distance of about 125cm. Assuming a shoulder height of 1.5 metres for the grizzly bear, therefore arms 1.5 metres long, which can easily deliver enough force to kill you while fully extended, this becomes a problem for the enemy.

So maybe the other guys think this through and decide to use a bow and arrow, after all, it is a big target, problem is, arrows don't pierce steel all that well, and even if it did make it through the armour, it now has to make it through a thick layer of fur, a thick layer of tough bear skin, and a thick layer of fat, before it can do any real damage.

As far as I can see, this only really leaves three alternatives, another bear, an elephant, or fire. Elephants are even more logistically difficult than bears, and not all that common in war, when it comes to other bears, it would likely come down to the bigger, or better armoured bear.
As for fire, there is a reason it wasn't used all that commonly in medieval open warfare, when you have a massive tract of land, packed tight with men, trampled grass, and beasts of war, a fire can be difficult to control, and could just as easily end up killing your own men as it could the enemy.

As for logistics, I have a couple of things I would like to point out.

First off, as "Thomas Pornin" pointed out in an earlier post, grizzly bears eat 75% plant matter, berries, roots and whatnot.
Second of all, even if a bear cost as much to feed as ten men, it could well still be worth it, as they could easily be worth more than ten men on the battlefield, especially when used at the right times.
And third of all, even if you were not willing to feed fallen men of the other side to your bears, which could be for any number of reasons, you could still feed the enemies fallen beats of burden. The horses they ride, and the cattle and donkeys used to drag their supply wagons.
My final point on logistics, is that if you were to send the bears ahead for scouting, some time could be dedicated to allowing the bear to forage, or even hunt, as this would lighten the load on your supplies.

When it comes to tactics, there are several purposes a bear could fulfill.

As scouts, bears might suprise you with how quietly they could make their way through the woods. They would also easily cover ground most people may have trouble with, and would be able to hold their own if they encountered an enemy scout. Obviously you would still need to have at least one person with them, to relay gathered information and whatnot.

On the battleground, while they would be powerful weapons, what would truly make them valuable, would be the fact that your enemy knows they need to be killed, and this will take a large number of soldiers to accomplish. And while twenty enemy men reach your front line concentrated on killing the bear, they are left relatively open to a counter attack by your more traditional forces. Allowing you to use the bear as a sort of anvil, and your men as the hammer.

As a front line unit, they would likely be invaluable to holding the line, as they would be near impossible to push back, even by a cavalry charge, as they are heavier and stronger than even traditional heavy cavalry, and likely wouldn't have much of an issue breaking a horses neck.

They could also serve as a kind of heavy cavalry, accompanied by handlers rather than ridden. The impact they had on the front line of the enemy would be considerable, and quite probibly measured by the tens of enemies fallen per bear.

During any kind of siege, they can take on an additional purpose, properly equipped they could be turned into a kind of living battering ram, generating more force on impact than a team of men.

Additionally, they are, despite their size, capable climbers, and could scale low walls erected by the enemy.

However their is an alternative you could use for your fictional world, which could quite probibly fix any logistics problem you encounter. Giant ground sloths.

These varied in size depending on the species, from the size of a large ape, to the size of a small Elephant, and we're built much like bears.

This solution providers a couple of pros and cons, listed below.

They where herbivores, meaning they where easy to feed, they could simply browse on trees as they went.

They had a network of bones under their considerable hide, which served a perfective purpose, and have been compared to chainmail.

Well normally placid, and thus easily controlled along the journey, unlike horses, they are built for fight, not flight. This means that during battle they would instinctively swipe at the guys pointing the pointy things at them.

However, they still would not have made the best mounts, as, due on part to their large claws, were probably knuckle walkers.

They were also likely considerably slower than a bear of comparable size, they simply weren't built for speed.

As for actually getting to the point where you could take one of these animals into battle, well, a ground sloth likely lived in small groups and so may be easier to tame and domesticate, but as suggested by other posters, you could have the bears in your world be social creatures, it's really up to you.

I'm surprised that, out of all the wonderful answers given so far, nobody mentioned a technological solution.

Cyborg Bears could be guided, and even controlled by their brain implants. Perhaps their anatomy could even be modified to make them easier to ride or to give them enhanced senses, weapon mounts, etc.

Of course, one might wonder why such a technologically advanced society might use cyborg bear cavalry instead of regular vehicles...

Technology developed along different lines than in our world. Just
because we invented things in a certain pattern doesn't mean everyone
will.

The society in question may have lost the ability to build war vehicles and other similar tech.

They may not have the ability to fuel vehicles, while still having power sources for computers (biochemically powered implants). Perhaps von Neumann (self replicating) technology is at the heart of their computing tech.

The society may have a cultural or religious taboo against vehicles.

Environmental factors may make using living mounts more practical than vehicles. Perhaps they have wheeled vehicles, but nothing that can handle rough terrain. Perhaps some form of radiation prevents the use of their regular transportation technology.

The Bear Cavalry serves a ceremonial purpose more than a practical one.

Of course, there's always magic to modify and control the bears as well.

Animals such as bears are a large competitor for prey animals and require a lot of meat to keep happy. You don't want your ride to decide that you will make a nice snack. Keeping a large bear well fed is very expensive, that is why so many are in shelters or have to be put down, they cost too much to keep, and that is as a pet with an easy life, not a hard working war mount.

Bears are also usually solitary, partly because it is easier to feed fewer mouths when you get bigger. Wolves are pack animals and a human can replace a wolf as the leader. This will not with bears, there is no such thing as a bear pack (thank god).

Most animals we ride are herd animals, they are easy to feed, usually grass and other vegetation (often stuff we can't digest well) so there is no competition for food and they are generally not going to try and kill each other.